Computers include general purpose central processing units (CPUs) that are designed to execute a specific set of system instructions. A group of processors that have similar architecture or design specifications may be considered to be members of the same processor family. Examples of current processor families include the Motorola 680X0 processor family, manufactured by Motorola, Inc. of Phoenix, Ariz.; the Intel 80X86 processor family, manufactured by Intel Corporation of Sunnyvale, Calif.; and the PowerPC processor family, which is manufactured by Motorola, Inc. and used in computers manufactured by Apple Computer, Inc. of Cupertino, Calif. Although a group of processors may be in the same family because of their similar architecture and design considerations, processors may vary widely within a family according to their clock speed and other performance parameters.
Each family of microprocessors executes instructions that are unique to the processor family. The collective set of instructions that a processor or family of processors can execute is known as the processor's instruction set. As an example, the instruction set used by the Intel 80X86 processor family is incompatible with the instruction set used by the PowerPC processor family. The Intel 80X86 instruction set is based on the Complex Instruction Set Computer (CISC) format. The Motorola PowerPC instruction set is based on the Reduced Instruction Set Computer (RISC) format. CISC processors use a large number of instructions, some of which can perform rather complicated functions, but which require generally many clock cycles to execute. RISC processors use a smaller number of available instructions to perform a simpler set of functions that are executed at a much higher rate.
The uniqueness of the processor family among computer systems also typically results in incompatibility among the other elements of hardware architecture of the computer systems. A computer system manufactured with a processor from the Intel 80X86 processor family will have a hardware architecture that is different from the hardware architecture of a computer system manufactured with a processor from the PowerPC processor family. Because of the uniqueness of the processor instruction set and a computer system's hardware architecture, application software programs are typically written to run on a particular computer system running a particular operating system.
Computer manufacturers want to maximize their market share by having more rather than fewer applications run on the microprocessor family associated with the computer manufacturers' product line. To expand the number of operating systems and application programs that can run on a computer system, a field of technology has developed in which a given computer having one type of CPU, called a host, will include an emulator program that allows the host computer to emulate the instructions of an unrelated type of CPU, called a guest. Thus, the host computer will execute an application that will cause one or more host instructions to be called in response to a given guest instruction. Thus the host computer can both run software design for its own hardware architecture and software written for computers having an unrelated hardware architecture. As a more specific example, a computer system manufactured by Apple Computer, for example, may run operating systems and program written for PC-based computer systems. It may also be possible to use an emulator program to operate concurrently on a single CPU multiple incompatible operating systems. In this arrangement, although each operating system is incompatible with the other, an emulator program can host one of the two operating systems, allowing the otherwise incompatible operating systems to run concurrently on the same computer system.
When a guest computer system is emulated on a host computer system, the guest computer system is said to be a “virtual machine” as the guest computer system only exists in the host computer system as a pure software representation of the operation of one specific hardware architecture. The terms emulator, virtual machine, and processor emulation are sometimes used interchangeably to denote the ability to mimic or emulate the hardware architecture of an entire computer system. As an example, the Virtual PC software created by Connectix Corporation of San Mateo, Calif. emulates an entire computer that includes an Intel 80X86 Pentium processor and various motherboard components and cards. The operation of these components is emulated in the virtual machine that is being run on the host machine. An emulator program executing on the operating system software and hardware architecture of the host computer, such as a computer system having a PowerPC processor, mimics the operation of the entire guest computer system.
The emulator program acts as the interchange between the hardware architecture of the host machine and the instructions transmitted by the software running within the emulated environment. This emulator program may be a host operating system (HOS), which is an operating system running directly on the physical computer hardware. Alternately, the emulated environment might also be a virtual machine monitor (VMM) which is a software layer that runs directly above the hardware and which virtualizes all the resources of the machine by exposing interfaces that are the same as the hardware the VMM is virtualizing (which enables the VMM to go unnoticed by operating system layers running above it). A host operating system and a VMM may run side-by-side on the same physical hardware.
Within any computer operating system (OS), whether a host OS or guest OS, there exists one or more software stacks, each of which is a set of layered programs through which an application program may communicate to lower-level hardware. For example, software stacks typically exist for handling disk operations, video (i.e., graphics) operations, and networking operations. Within each stack, the higher-level components talk to the lower-level components and vice versa; i.e., each layer talks to the one above and below it.
An application programming interface (API) is an example of a higher-level stack component. An API is a set of routines, tools, and protocols used by software developers to write applications that are compatible with a specific operating system or interface. By contrast, a device driver is an example of a lower-level stack component. A device driver is a program that controls a particular type of peripheral device that is attached to a computer. There are device drivers for printers, displays, CD-ROM readers, diskette drives, and so on. Furthermore, several other software layers may exist between the highest and lowest levels of the software stack.
In the case of a graphics stack, for example, a 3D API is an example of a high-level component of the stack. This generic term refers to any API that supports the creation of standard 3D objects, lights, cameras, perspectives, and so on. Such APIs include Argonaut's BRender and Microsoft's Reality Lab. Beneath the 3D API in the graphics stack there may be, for example, a graphics device interface (GDI), which is the software layer that handles the interaction between application programs and the hardware of the display adapter. GDI translates a program's request to, for example, draw a line between two points into the pattern of pixel changes that the display adapter expects. Finally, the lowest-level component of the graphics stack may be, for example, a video card device driver, which is the software layer that communicates to the low-level computer hardware, i.e., the video display device, such as a computer monitor.
Continuing with the example of a graphics application, in today's virtual machine environment, the command progression is from the guest OS application, through the guest OS graphics stack to form a bitmap for the synthetic virtual graphics hardware. The VMM then receives this bitmap and reconstructs it into a graphics call for the graphics stack of the host OS. This graphics call is then passed from the VMM to the host OS and through the host OS graphics stack to form a bitmap for the physical graphics hardware.
However, in situations where the graphics stack for the guest OS and the graphics stack for the host OS are similar (or at least compatible), this through-back-through approach is three-times redundant—that is, a graphics call must travel through three essentially redundant layers, making one largely unnecessary round-trip, to finally draw the bitmap to the hardware display device. As a result, there are inefficiencies inherent in the operation of today's virtual machines, whereby much overhead is incurred by the redundant processing of commands through similar or identical guest OS and host OS software stacks. What is needed is a way to eliminate the redundant processing of commands through similar software stacks in both the guest OS and host OS to thereby achieve a more efficient virtual machine environment.